1. Field of the Invention
This invention pertains generally to telecommunications, and particularly to returning a user equipment unit to idle mode after a reset of a control node of a radio access network.
2. Related Art and Other Considerations
In a typical cellular radio system, wireless user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks. The user equipment units (UEs) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network. Alternatively, the wireless user equipment units can be fixed wireless devices, e.g., fixed cellular devices/terminals which are part of a wireless local loop or the like.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller node (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks. The core network has service domains, with an RNC having an interface to these service domains.
One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestial Radio Access Network (UTRAN). The UMTS is a third generation system which in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN is essentially a radio access network providing wideband code division multiple access (WCDMA) to user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM-based radio access network technologies.
Other types of telecommunications systems which encompass radio access networks include the following: Advance Mobile Phone Service (AMPS) system; the Narrowband AMPS system (NAMPS); the Total Access Communications System (TACS); the Personal Digital Cellular (PDS) system; the United States Digital Cellular (USDC) system; and the code division multiple access (CDMA) system described in EIA/TIA IS-95.
The topology of a radio access network can be conceptualized in areas or units larger than cells. Taking the UTRAN as an example radio access network, a UTRAN Routing Area (URA) is a geographical area comprising one or more cells. Each URA is identified by a unique identity which is broadcast in all cells belonging to the URA. A URA can comprise cells controlled by more than one RNC. A URA with more cells in more than one RNC is overlapping between the RNCs, i.e. an overlapping URA.
As another example from UTRAN, a Location Area (LA) is a geographical area comprising one or more cells. Each LA is identified by a unique identity sent on the broadcast channel, in the same way as the URA. However, a location area is used by the core network to track the location of the UE (in idle mode and in connected mode), while the URA is used by the radio access network to track the location of the UE in connected mode. Typically, a location area is geographically larger than a URA. To each location area there is one of several RNCs having cells in that particular location area. A relationship between location area and RNC is stored in the core network.
There are several interfaces of interest in the UTRAN. The interface between the radio network controllers (RNCs) and the core network(s) is termed the “Iu” interface. The interface between a radio network controller (RNC) and its base stations (BSs) is termed the “Iub” interface. The interface between the user equipment unit (UE) and the base stations is known as the “air interface” or the “radio interface” or “Uu interface”. In some instances, a connection involves both a Serving or Source RNC (SRNC) and a target or drift RNC (DRNC), with the SRNC controlling the connection but with one or more diversity legs of the connection being handling by the DRNC. An Inter-RNC transport link can be utilized for the transport of control and data signals between Source RNC and a Drift or Target RNC, and can be either a direct link or a logical link. An interface between radio network controllers (e.g., between a Serving RNC [SRNC] and a Drift RNC [DRNC]) is termed the “Iur” interface.
The radio network controller (RNC) controls the UTRAN. In fulfilling its control role, the RNC manages resources of the UTRAN. Such resources managed by the RNC include (among others) the downlink (DL) power transmitted by the base stations; the uplink (UL) interference perceived by the base stations; and the hardware situated at the base stations.
Those skilled in the art appreciate that, with respect to a certain RAN-UE connection, an RNC can either have the role of a serving RNC (SRNC) or the role of a drift RNC (DRNC). If an RNC is a serving RNC (SRNC), the RNC is in charge of the connection with the user equipment unit (UE), e.g., it has full control of the connection within the radio access network (RAN). A serving RNC (SRNC) is connected to the core network. On the other hand, if an RNC is a drift RNC (DRNC), its supports the serving RNC (SRNC) by supplying radio resources (within the cells controlled by the drift RNC (DRNC)) needed for a connection with the user equipment unit (UE). A system which includes the drift radio network controller (DRNC) and the base stations controlled over the Iub Interface by the drift radio network controller (DRNC) is herein referenced as a DRNC subsystem or DRNS.
Radio access networks typically have a particular signalling protocol employed between the radio access network and the user equipment unit to support the management of radio resources. For example, UTRAN has its Radio Resource Control (RRC) layer 3 signalling protocol. A user equipment unit in the RRC protocol operates in a state model conceptualized as having two modes: an Idle Mode and a Connected Mode. The Idle Mode is entered after power on. In Idle Mode there is no connection between the user equipment unit (UE) and the UTRAN. When an RRC connection is established, the user equipment unit (UE) is assigned a U-RNTI and the user equipment unit (UE) enters Connected Mode. The U-RNTI (UTRAN Radio Network Temporary Identity) is a global identity, which can be used in any cell in the UTRAN. In Connected Mode, the RNC in charge of the RRC connection for this UE is denoted as the Serving RNC (SRNC). The U-RNTI consists of two parts: the SRNC-identity (which within UTRAN identifies the SRNC for this UE) and the Serving RNTI (S-RNTI) which identifies the RRC connection within the particular SRNC.
As illustrated by FIG. 8, within Connected Mode there are four different states: CELL_DCH state; CELL_FACH state; CELL_PCH state; and URA_PCH. As briefly summarized, each state reflects a different level of activity. In the CELL_DCH state a dedicated control channel (DCCH) is used for transmission of signalling messages between the user equipment unit (UE) and the UTRAN. In the CELL_FACH state, no dedicated physical channel is assigned, but the user equipment unit (UE) listens continuously to a common channel (the FACH) in the downlink belonging to the selected cell. In the uplink for the CELL_FACH state, the user equipment unit (UE) typically uses a random access channel (RACH). At each cell reselection, the user equipment unit (UE) updates the network with its current cell location. In the CELL_PCH state, the user equipment unit (UE) monitors a paging channel (PCH) of a selected cell. In the CELL_PCH state the user equipment unit (UE) updates the network with its current cell location at cell reselection. On the PCH, means for addressing individual user equipment units (UEs) exist (using the U-RNTI), but the user equipment unit (UE) can not transport any signalling messages to the network. The URA_PCH state resembles the CELL_PCH state, but primarily differs in that the user equipment unit (UE) only updates the network of its location after crossing UTRAN Routing Area (URA) borders.
In making or attempting to make connection with a user equipment unit operating in a radio access network, a core network typically assigns a core network UE identity (e.g., CN UE identity) to the user equipment unit. But the CN UE identity assigned to a particular user equipment unit may not be unique CN UE identity when used on a common transport channel to a connected mode UE. Keep in mind that there may be several core networks which are assigning CN UE identities. It may turn out, for example, that an idle mode UE registered in different location and/or routing area may camp in the same cell as with a connected mode UE, with both the idle mode UE and connected mode UE having been assigned the same CN UE identity. The typical scenario when such common assignment may happen is when the connected mode UE camps in a cell controlled by a drift RNC (DRNC). If the DRNC receives a paging message from a CN node, intended to the idle mode UE, it should be able to use the CN UE identity as the identity when paging the idle mode UE without a risk that the connected mode UE may respond to the page.
In order to avoid such confusion, an RNC which receives a page from the core network handles the page differently depending on whether the RNC has an established RRC connection the given UE or not. For example, if the UE to be paged is in connected mode, the connected mode UE is addressed for paging purposes using the U-RNTI (described above). On the other hand, if the UE to be paged is in idle mode, the idle mode UE is addressed for paging purposes using the CN UE identity assigned to the idle mode UE.
An RNC generally has interfaces to base stations, interfaces to other RNCs, diversity handover functionality, and other components and functional units employed, e.g., in conjunction with the radio resource control protocol. Various functionalities of the RNC are preformed by several processors which are running (executing) different processes and/or handling different RRC connections. Typically, the control of the UE connections owned by the RNC is shared or partitioned among the processors, so that one processor handles only a part, group, or subset of the UEs having connections controlled by the RNC.
There are times at which the RNC of a radio access network must undergo a RNC “reset” (also known as a RNC “restart”). As explained subsequently below, RNC reset can have considerable ramifications, including but not limited to ramifications for paging of UEs.
There are two basic types of situations in which RNC reset occurs. The first type of situation is a failure of the RNC itself, discussed in more detail below. The second type of situation is propagated from a core network node (e.g. MSC, SGSN) which had a failure and which transmits a RESET message to the RNC. In this second type of situation, the RNC deletes the information stored for all UE connections (including bringing the UEs to idle mode) and replies to the CN node with a RESET ACKNOWLEDGE message.
There are several possible causes of the first type of situation, i.e., failure of the RNC causing RNC reset. For example, hardware glitches may typically affect one or a few of the processors comprising the RNC. As another example, software upgrades (typically made on a processor by processor basis) may require reboot of the processor involved in the upgrade. But perhaps the most common cause of RNC reset is a problem discovered in execution of software by one of the processors of the RNC. The execution problem may require remedial action such as a reboot, for example. Typically in such failure it is sufficient to restart just the processor where the failure was detected, and then propagate clean-up operations to any other processors that may be dependent on the processor that restarted. In other less drastic cases, in which the software fault can be isolated to just one particular process performed by a processor, the processor itself may not need to be restarted just because the particular process crashed, but the process itself must be restarted (but nevertheless essentially causing a RNC reset). In more serious cases it may be detected that repeated restarts of a problematic processor do not solve the problem, thereby requiring restart of all processors of the RNC as the recovery measure.
Whatever the cause or situation requiring reset, it is a problem that, upon being reset, a control node of a radio access network, such as an radio network controller (RNC) of the UTRAN, may lose certain information about the context of the user equipment unit, known as the “UE context” in the UTRAN.
The information included in UE context comprises, among others, the following parameters: IMSI (the international mobile subscriber identity); C-ID; D-RNTI; and RNC Identity of the DRNC where the user equipment unit (UE) is currently located. The international mobile subscriber identity (IMSI) [which comprises not more than fifteen digits] comprises three components: a mobile country code (MCC) [three digits]; a mobile network code (MNC) [two or three digits]; and a mobile subscriber identification number (MSIN). The D-RNTI parameter is similar to S-RNTI parameter, but identifies the UE context information in the DRNC. The C-ID parameter is the Cell Identity of where the UE is currently located. The C-ID parameter is not applicable to the UEs in the URA_PCH state, since the location of a user equipment unit (UE) in the URA_PCH state is not known to the cell level, but rather is known on URA level (a group of cells defined as one URA). With regard to the RNC Identity parameter, it is noted that in the Cell_DCH state there could be many simultaneous radio links (RLs), so there could conceivably be as many RNCs (at least theoretically) handling legs of connections to the UE.
Consider, for example, the case of a connected mode UE whose UE context was lost during RNC reset. Loss of the UE context necessarily means loss of the U-RNTI for that connected mode UE. One problem is that the connected mode UE expects to be addressed using the (now lost) U-RNTI. So in view of the loss of UE context and U-RNTI in particular, paging of the connected mode UE may be ineffective until that UE goes back to idle mode.
A typical scenario occurs when a UE camps in a cell that is controlled by one RNC (the “DRNC”), while the control of the radio connection for this particular UE is handled by a different RNC (the “SRNC”). In case of a reset of the SRNC, all DRNCs need to release the radio connections for all UEs having connections which were controlled by the reset SRNC and which are camping on cells controlled by the DRNCs.
The present conventional mechanism for aligning the resources of two RNCs (RNC1 and RNC2) in the event of an abnormal failure and for releasing radio connections in such scenario is awkward and inefficient. In accordance with this mechanism, after reset of a particular RNC, the reset RNC transmits RESET REQUEST messages to all neighboring RNCs (which may serve as DRNCs for the UEs that had the RNC that was reset as their SRNC). In the RESET REQUEST message there is a possibility either to address “all” UEs or to provide a list of UE identities. The DRNC is expected to release all radio connections that were addressed in the RESET REQUEST message and then return a RESET RESPONSE message to the SRNC.
As a practical matter, in accordance with the present mechanism the DRNC has to release the RRC connections one by one (from the S-RNTI list in the RESET REQUEST message or all S-RNTIs). This release of RRC connections one by one takes considerable time, employs extensive signaling, and runs the risk that before a given RRC connection is released a page to that UE involved in that RRC connection may be lost.
What is needed, therefore, and an object of the present invention, is a more effective way to communicate the fact of RNC reset.